Compound loudspeaker diaphragm



Oct. 20, 1964 J. F. NOVAK COMPOUND LOUDSPEAKER DIAPHRAGM 2 Sheets-Sheet 1 Filed Aug. 30. 1961 Oct. 20, 1964 J. F. NOVAK 3, 53, 63

Filed Aug. 50. 1961 2 Sheets-Sheet 2 James i i M W q JAM Oys:

United States Patent O 3,153,463 COMPGUND LOUDSPEAKER DIAPHRAGM ames F. Novak, 'La Grange Park, lil., assgnor to The Muter Company, Chicago, lil., a corpo'atron of Iliinois Filed Aug. 30, 1961, Ser. No. 135,()01 11 Claims. (CI. 181--32) This invention relates to loudspeaker diaphragms, and more particularly refers to a diaphragm having a short axial dimension.

Except for the smallest units, loudspeaker diaphragms have traditionally been constructed in conical form. The conical form has been used since it increases the rigidity of the diaphragm, while at the same time permitting thin light-weight material such as paper to be used in its fabrication. One of the disadvantages of conical diaphragms is that they require a relatively large axial space.

Attempts have been made to reduce the aXial dimensions of loudspeakers in order that thin acoustical enclosures which are more compatible with modern furniture designimight be used to house the speakers. As a means for reducing the axial dimensione, fiat diaphragms have to some extent been utilized, even for speakers designed to reproduce low frequency sound. However, when flat diaphragms are driven in the usual manner by a voice coil mounted at the center of thediaphragm, it has been found that the vibrating diaphragm breaks up into several vibrational modes, with the end result that severe distortion is introduced.

It is an object of theinvention to provide a loudspeaker diaphragm which, being composed of two substantially flat parts, is substantially flat but which, nevertheless, will reproduce sound without undue distortion.

It is a further object to provide such a diaphragm i which can be used with speakers having small axial dimensons.

It is a further object to provide such a diaphragm which is light in Weight and yet rigid.

It is further an object to provide a flat diaphragm which vibrates as a piston without breaking up into several vibrational modes.

It is still further an object to provide a diaphragm as described which is relatively simple and inexpensive to produce.

Other objects and advantages of the invention will become apparent from the following discussion and the accompanying drawings in which:

FIG. 1 is a cross-sectional view of a daphragm em bodying the present invention;

FIG. 2 is a cross-sectio nal view of a diaphragm according 'to the invention utilizing a somewhat modified voice coil attachment;

FIG. 3 is a cross-sectional view of an alternative embodiment of the invention, and

FIG. 4 is a rear View of a portion of the structure shown in FIG. 3. j "4 According to'the invention ithas been found that a rela'tively flat and rigid air-moving means which will vibrate as a pisten without breaking up into several vibrational modes may be provided by utilizing a compound diaphragm having a fiat main air-moving' diaphragm member cooperating with an auxilary driving member aiiixed thereto at the periphery. The auxiliary driving member operates to transmit vibrations from the voice coil, which is mounted at the axis of the driving member, to` the periphery of the fiat diaphragrn member.

%53,453 Patented Oct. 20, 1954 tially fiat driving member 3 is arranged to transmit motion to the periphery of the diaphragm member through the shoulder 2. The functions of the shoulder 2 are to maintain the major portion of the inner surface of the diaphragm 1 spaced apart from the major portion of the surface of the driving member 3, and to provide a means for transmitting driving force to the diaphra gm from the driving member. A fiexible annular ring 4 may be mounted between the diaphragm 1 and the driving member 3 at the peripheral shoulder to support the diaphragm assembly. The periphery of the annular ring 4 is in turn afiixed to the loudspeaker frame, not shown.

Mounted centrally at the aXial aperture 3' of the driving member 3, as by cementing, is a voice coil bobbin S having a voice coil 6 mounted thereon. A channel 7 connects the voice coil opening with the space la between the diaphragm 1 and the driving member 3.

The structure at the periphery of the diaphragm assemby may have a somewhat altered arrangement. Instead of having the shoulder at the periphery of the diaphragm for the application of the driving force, the diaphragm periphery may be flat and a separate ring may be cemented to and positioned between the diaphragm and the annular ring 4, or the annular mounting ring may itself serve to provide the driving connection between the two members. Alternatively, the annular ring 4 may be directly aixed to a flat diaphragm and an extension of the driving member used to separate the diaphragm from the driving member. Whatever arrangement is used, it is important that the driving force be applied to the diaphragm mainly at an annular area near its periphery to avoid the breaking up of the diaphragm into separate vibrational modes.

A somewhat modified form of the compound diaphragm assembly is shown in FIG. 2. Here, as in FIG. 1, the diaphragm 8 is driven at its periphery 9 by a substantially fiat driving member 10. An annular supporting ring 11 is inserted between the two parts in a manner similar to that of FIG. l. In this arrangement, the driving member 10, instead of having an axial opening or channel connecting with the space 'la between the diaphragm and driving member, as in FIG. 1, has only a shallow well 10' provided therein into which the bobbin 12 supporting the voice coil 13 is cemented. The operation of this structure is similar to' that of the device of FIG. 1, except that here the air between the diaphragm and the driving member is entrapped to form an elastic cushion with damping qualities.

Among the materials contemplated for use in the present invention are the class of materials known in the art as rigid plastic foams or rigid expanded plastics.

Such materials are prepared by incorporating a small amount of blowing agent into a fusible or liquid plastic or resinous material, either while the material is in the liquid stage, or, in the case of bead-form materials, by causingthe blowing agent to be absorbed into the individ ual beads. Under the proper conditions the blowing agent causes the plastic material to eXpand by the reacting therewith' or with water liberated therefrom to form a gas, by reaction of two or morecomponents of the blowing agent, or by the vaporization or decomposition of the blowing `agent into a gas when the temperature of the foamable material, is raised. After the plastic material has been caused to expand to the desired den sity, it is solidified, either' by cooling, as in'the case of thermoplastic materials, or by curing or setting in the case of thermosetting materials. The resulting product hasa I structure comprised of a multiplicityof very fine, uniform, i

discrete closed cells, eachbeing a miniature bubble with a very thin plastic wall which is common to that .of adwhich may be used to prepare rigid foams suitable for the present invention are: polystyrene, polyethylene, polyvinyl chlorde, phenol-formaldhyde condensation products, various polyurethane resins, and many others. Suitable foaming agents for each of these materials are Well known in the art. Thermosetting plastic foams are entirely suitable for the practice of the present invention.

Polystyrene beads suitable for use in the present invention are commercially marketed by Koppers Company, Inc., under the trademark Dylite. This material is more fully described in British Patent #756,654, the disclosure of which is herein incorporated by reference.

The diaphragm and the 'driving member may be fabrcated by placing the required amount of polystyrene heads in a mold and applying heat by means of steam. The beads are caused to expand and to fill the molding form. Alternatively, the beads may be extruded into a flat sheet and 'the fiat diaphragm then directly cut from the sheet.

As an alternative to the use of plastic foam for the daphragm assembly, the diaphragm, as Well as the drivin'g member, may be fabricated from paper or other sheet form material which is stitened by means of a resir or sizing agent. In the case of the driving member, increased rigidity is attained by molding radial ribs in the structure body. Such a structure is illustrated in FTGS. 3 and 4. Here the diaphragm 14 is comprised of a fiat sheet of paper which is made rigid by impregnation With a lacquer. The diaphragm 14 is driven at its periphery 15 by stiff substantially flat driving member 16 which is composed of a felted and lac'quered paper having radial ribs 17 molded thereinto. The paper used for the supporting ring 18 is also preferably impregnated With a lacquer, as by dippng.

p A voice coil bobbin 19 hearing a voice coil 20 is cemented to the driving member 16.

Various combinations of the structures shown in the drawings may be used. For example, a foamed plastic driving member may be used with a stiffen'ed paper diaphragm, or with a molded foam diaphragm; or a paper driving member may be used with a paper diaphragm or a foamed plastic diaphragm. In every case, because the diaphragm is driven substantially at its periphery, it does not break up into vibrational modes.

The d'riving member itself, because it is driven near its center and is substantially flat, will tend to break up into vibrational modes to a limited degree. However, this etfect is substantially damped out and, in'any event does not m'anifest itself in the sound reproduced by the diaphragm unit because the diaphragm, being driven only at its perip'hery by the pe'ripheral portion of 'the driving member, does not to any appreciable degree break up into vibrational modes. Moreover, the sound' waves from the back of the driving member are substantially 'attenuated by the acoustical enclosure in which the speaker is mounted.

Other rib or reinforcement patterns may be used as alternatives to the pattern shown in the drawings, the primary requirement being maximum radial stiffness 'of the driving member.

The double diaphragm assembly of the presen'tinvention has a number of advantages over conical diaphra'gms. First, it has small axia'l dimension', permitting unusually slimcabinetry to be utili'zed. Second, because the radiator is in the form of a flat plane, 'the entire surface is effective for eflieient 'sound production and radiation. Third, because thepiston-li'ke diaphragm is driven at least main'ly at its periphery, the tendercy for the diaphragm to vib'rate in spurious vibrational modes is suppressed. The diaphragm assembly is tsturdy, light and rig'id. It provides excellent emciency and improved tran'sient response. It has good tonal range and balance and 'is free from coloration.

In addition to its excellent response characteristics,

the compound diaphragm assembly of the invention is' skilled in the art without departing from the spirit and scope of the invention as defined in the appended claims.

Invention is claimed as follows:

1. A compound loudspeaker diaphragm assembly comprising a substantially flat and rigid diaphragm, a rigid substantially fiat driving member spaced except at the periphery thereof from said diaphragm and having its periphery atfixed to said diaphra gm only at substantially the periphery thereof, and a Voice coil coaxially mounted on said driving member, the diameter of said voice coil being substantially smaller than the diameter of said diaphragm.

2. A compound loudspeaker diaphragm assembly according to claim 1 wherein said diaphragm is composed of a rigid expanded plastic.

3. A compound loudspeaker diaphragm assembly according to claim l wherein said driving member is composed of a rigid expanded plastic.

4. A compound loudspeaker diaphragm assembly according to claim 1 wherein both said diaphragm and said driving member are composed of a rigid expanded plastic.

' izing material, and having radial reinforcing ridges molded theren.

7. A compound loudspeaker diaphragm assembly according to claim 1 wherein both said diaphragrn and said driving member are composed of fibrous sheet material impregnated with a resinous rigidizing material, said driving 'member "having radial reinforcing ribs molded therein.

8. A compound loudspeaker diaphragm assembly comprising a substantially flat and rigid diaphragm, a substantially flat and rigid drivin'g member spaced except at the periphery thereof from said daphragm and having its periphery afi'ixed to said diaphragm only at substantially the periphery 'thereofl a voice coil mounted on said drivng member coaxially therewith, the diameter of said voice coil being substantially smaller than the diameter of said diaphragm, and a fiexible annular suspension member afiixed to said diaphragm assembly at the periphery thereof.

9. A compound loud'speaker diaphragm assembly comprising a substantially fiat and rigid diaphragm, a substantially fiat and rigid driving member spaced except at the periphery thereof from said di'aphragm and having its periphery affixed to saiddiaphragm only at substantially the periphery thereof, said driving member having a channel therein coaxial therewith, a voice coil bobbin Secured to said driving member in said channel, and a voice. coil mounted on said bobbin, the diameter of said voice coil being substantially smaller than the diameter of said diaphragm. p

10. A compound loudspeaker diaphragm assembly ccmprising a substantially fiat and rigid diaphragm, a substantially fiat and rigid driving member spaced except at the peripherythereoffrom said diaphragm and having its periphery afxed to said diaphragm only at substantially the periphery thereof, said driving member having a well extending thereinto coaxially therewith, a voice coilbobbin extending into said well and aixed to said driving member, and a voice coil on said bobbin, the diameter of said voice coil being substantially smaller than the diameter of said diaphragm. i

11. A compound loudspeaker diaphragm assembly comprising a substantially fiat and rigid diaphragm, a substantially flat and rigid driving member having its periph- 5 ery affixed to said diaphragn at the entire periphery thereof but being spaced apart from said diaphragm throughout the major portion of the area thereof to define a closed space therebetweer, and a voice coil coaxally mounted on said driving member, the diameter of said voice coil being substantially smaller than the diameter of said daphragm.

References Cite in the file of this patent UNITED STATES PATENTS 1,644,605 Pavek Oct. 4, 1927 

1. A COMPOUND LOUDSPEAKER DIAPRAGM ASSEMBLY COMPRISING A SUBSTANTIALLY FLAT AND RIGID DIAPHRAGM, A RIGID SUBSTANTIALLY FLAT DRIVING MEMBER SPACED EXCEPT AT THE PERIPHERY THEREOF FROM SAID DIAPHRAGM AND HAVING ITS PERIPHERY AFFIXED TO SAID DIAPHRAGM ONLY AT SUBSTANTIALLY THE PERIPHERY THEREOF, AND A VOICE COIL COAXIALLY MOUNTED ON SAID DRIVING MEMBER, THE DIAMETER OF SAID VOICE COIL BEING SUBSTANTIALLY SMALLER THAN THE DIAMETER OF SAID DIAPHRAGM. 